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Linux/Documentation/filesystems/debugfs.rst

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Diff markup

Differences between /Documentation/filesystems/debugfs.rst (Version linux-6.11.5) and /Documentation/filesystems/debugfs.rst (Version linux-6.1.114)


  1 .. SPDX-License-Identifier: GPL-2.0                 1 .. SPDX-License-Identifier: GPL-2.0
  2 .. include:: <isonum.txt>                           2 .. include:: <isonum.txt>
  3                                                     3 
  4 =======                                             4 =======
  5 DebugFS                                             5 DebugFS
  6 =======                                             6 =======
  7                                                     7 
  8 Copyright |copy| 2009 Jonathan Corbet <corbet@l      8 Copyright |copy| 2009 Jonathan Corbet <corbet@lwn.net>
  9                                                     9 
 10 Debugfs exists as a simple way for kernel deve     10 Debugfs exists as a simple way for kernel developers to make information
 11 available to user space.  Unlike /proc, which      11 available to user space.  Unlike /proc, which is only meant for information
 12 about a process, or sysfs, which has strict on     12 about a process, or sysfs, which has strict one-value-per-file rules,
 13 debugfs has no rules at all.  Developers can p     13 debugfs has no rules at all.  Developers can put any information they want
 14 there.  The debugfs filesystem is also intende     14 there.  The debugfs filesystem is also intended to not serve as a stable
 15 ABI to user space; in theory, there are no sta     15 ABI to user space; in theory, there are no stability constraints placed on
 16 files exported there.  The real world is not a     16 files exported there.  The real world is not always so simple, though [1]_;
 17 even debugfs interfaces are best designed with     17 even debugfs interfaces are best designed with the idea that they will need
 18 to be maintained forever.                          18 to be maintained forever.
 19                                                    19 
 20 Debugfs is typically mounted with a command li     20 Debugfs is typically mounted with a command like::
 21                                                    21 
 22     mount -t debugfs none /sys/kernel/debug        22     mount -t debugfs none /sys/kernel/debug
 23                                                    23 
 24 (Or an equivalent /etc/fstab line).                24 (Or an equivalent /etc/fstab line).
 25 The debugfs root directory is accessible only      25 The debugfs root directory is accessible only to the root user by
 26 default. To change access to the tree the "uid     26 default. To change access to the tree the "uid", "gid" and "mode" mount
 27 options can be used.                               27 options can be used.
 28                                                    28 
 29 Note that the debugfs API is exported GPL-only     29 Note that the debugfs API is exported GPL-only to modules.
 30                                                    30 
 31 Code using debugfs should include <linux/debug     31 Code using debugfs should include <linux/debugfs.h>.  Then, the first order
 32 of business will be to create at least one dir     32 of business will be to create at least one directory to hold a set of
 33 debugfs files::                                    33 debugfs files::
 34                                                    34 
 35     struct dentry *debugfs_create_dir(const ch     35     struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);
 36                                                    36 
 37 This call, if successful, will make a director     37 This call, if successful, will make a directory called name underneath the
 38 indicated parent directory.  If parent is NULL     38 indicated parent directory.  If parent is NULL, the directory will be
 39 created in the debugfs root.  On success, the      39 created in the debugfs root.  On success, the return value is a struct
 40 dentry pointer which can be used to create fil     40 dentry pointer which can be used to create files in the directory (and to
 41 clean it up at the end).  An ERR_PTR(-ERROR) r     41 clean it up at the end).  An ERR_PTR(-ERROR) return value indicates that
 42 something went wrong.  If ERR_PTR(-ENODEV) is      42 something went wrong.  If ERR_PTR(-ENODEV) is returned, that is an
 43 indication that the kernel has been built with     43 indication that the kernel has been built without debugfs support and none
 44 of the functions described below will work.        44 of the functions described below will work.
 45                                                    45 
 46 The most general way to create a file within a     46 The most general way to create a file within a debugfs directory is with::
 47                                                    47 
 48     struct dentry *debugfs_create_file(const c     48     struct dentry *debugfs_create_file(const char *name, umode_t mode,
 49                                        struct      49                                        struct dentry *parent, void *data,
 50                                        const s     50                                        const struct file_operations *fops);
 51                                                    51 
 52 Here, name is the name of the file to create,      52 Here, name is the name of the file to create, mode describes the access
 53 permissions the file should have, parent indic     53 permissions the file should have, parent indicates the directory which
 54 should hold the file, data will be stored in t     54 should hold the file, data will be stored in the i_private field of the
 55 resulting inode structure, and fops is a set o     55 resulting inode structure, and fops is a set of file operations which
 56 implement the file's behavior.  At a minimum,      56 implement the file's behavior.  At a minimum, the read() and/or write()
 57 operations should be provided; others can be i     57 operations should be provided; others can be included as needed.  Again,
 58 the return value will be a dentry pointer to t     58 the return value will be a dentry pointer to the created file,
 59 ERR_PTR(-ERROR) on error, or ERR_PTR(-ENODEV)      59 ERR_PTR(-ERROR) on error, or ERR_PTR(-ENODEV) if debugfs support is
 60 missing.                                           60 missing.
 61                                                    61 
 62 Create a file with an initial size, the follow     62 Create a file with an initial size, the following function can be used
 63 instead::                                          63 instead::
 64                                                    64 
 65     void debugfs_create_file_size(const char *     65     void debugfs_create_file_size(const char *name, umode_t mode,
 66                                   struct dentr     66                                   struct dentry *parent, void *data,
 67                                   const struct     67                                   const struct file_operations *fops,
 68                                   loff_t file_     68                                   loff_t file_size);
 69                                                    69 
 70 file_size is the initial file size. The other      70 file_size is the initial file size. The other parameters are the same
 71 as the function debugfs_create_file.               71 as the function debugfs_create_file.
 72                                                    72 
 73 In a number of cases, the creation of a set of     73 In a number of cases, the creation of a set of file operations is not
 74 actually necessary; the debugfs code provides      74 actually necessary; the debugfs code provides a number of helper functions
 75 for simple situations.  Files containing a sin     75 for simple situations.  Files containing a single integer value can be
 76 created with any of::                              76 created with any of::
 77                                                    77 
 78     void debugfs_create_u8(const char *name, u     78     void debugfs_create_u8(const char *name, umode_t mode,
 79                            struct dentry *pare     79                            struct dentry *parent, u8 *value);
 80     void debugfs_create_u16(const char *name,      80     void debugfs_create_u16(const char *name, umode_t mode,
 81                             struct dentry *par     81                             struct dentry *parent, u16 *value);
 82     void debugfs_create_u32(const char *name,      82     void debugfs_create_u32(const char *name, umode_t mode,
 83                             struct dentry *par     83                             struct dentry *parent, u32 *value);
 84     void debugfs_create_u64(const char *name,      84     void debugfs_create_u64(const char *name, umode_t mode,
 85                             struct dentry *par     85                             struct dentry *parent, u64 *value);
 86                                                    86 
 87 These files support both reading and writing t     87 These files support both reading and writing the given value; if a specific
 88 file should not be written to, simply set the      88 file should not be written to, simply set the mode bits accordingly.  The
 89 values in these files are in decimal; if hexad     89 values in these files are in decimal; if hexadecimal is more appropriate,
 90 the following functions can be used instead::      90 the following functions can be used instead::
 91                                                    91 
 92     void debugfs_create_x8(const char *name, u     92     void debugfs_create_x8(const char *name, umode_t mode,
 93                            struct dentry *pare     93                            struct dentry *parent, u8 *value);
 94     void debugfs_create_x16(const char *name,      94     void debugfs_create_x16(const char *name, umode_t mode,
 95                             struct dentry *par     95                             struct dentry *parent, u16 *value);
 96     void debugfs_create_x32(const char *name,      96     void debugfs_create_x32(const char *name, umode_t mode,
 97                             struct dentry *par     97                             struct dentry *parent, u32 *value);
 98     void debugfs_create_x64(const char *name,      98     void debugfs_create_x64(const char *name, umode_t mode,
 99                             struct dentry *par     99                             struct dentry *parent, u64 *value);
100                                                   100 
101 These functions are useful as long as the deve    101 These functions are useful as long as the developer knows the size of the
102 value to be exported.  Some types can have dif    102 value to be exported.  Some types can have different widths on different
103 architectures, though, complicating the situat    103 architectures, though, complicating the situation somewhat.  There are
104 functions meant to help out in such special ca    104 functions meant to help out in such special cases::
105                                                   105 
106     void debugfs_create_size_t(const char *nam    106     void debugfs_create_size_t(const char *name, umode_t mode,
107                                struct dentry *    107                                struct dentry *parent, size_t *value);
108                                                   108 
109 As might be expected, this function will creat    109 As might be expected, this function will create a debugfs file to represent
110 a variable of type size_t.                        110 a variable of type size_t.
111                                                   111 
112 Similarly, there are helpers for variables of     112 Similarly, there are helpers for variables of type unsigned long, in decimal
113 and hexadecimal::                                 113 and hexadecimal::
114                                                   114 
115     struct dentry *debugfs_create_ulong(const     115     struct dentry *debugfs_create_ulong(const char *name, umode_t mode,
116                                         struct    116                                         struct dentry *parent,
117                                         unsign    117                                         unsigned long *value);
118     void debugfs_create_xul(const char *name,     118     void debugfs_create_xul(const char *name, umode_t mode,
119                             struct dentry *par    119                             struct dentry *parent, unsigned long *value);
120                                                   120 
121 Boolean values can be placed in debugfs with::    121 Boolean values can be placed in debugfs with::
122                                                   122 
123     void debugfs_create_bool(const char *name,    123     void debugfs_create_bool(const char *name, umode_t mode,
124                              struct dentry *pa    124                              struct dentry *parent, bool *value);
125                                                   125 
126 A read on the resulting file will yield either    126 A read on the resulting file will yield either Y (for non-zero values) or
127 N, followed by a newline.  If written to, it w    127 N, followed by a newline.  If written to, it will accept either upper- or
128 lower-case values, or 1 or 0.  Any other input    128 lower-case values, or 1 or 0.  Any other input will be silently ignored.
129                                                   129 
130 Also, atomic_t values can be placed in debugfs    130 Also, atomic_t values can be placed in debugfs with::
131                                                   131 
132     void debugfs_create_atomic_t(const char *n    132     void debugfs_create_atomic_t(const char *name, umode_t mode,
133                                  struct dentry    133                                  struct dentry *parent, atomic_t *value)
134                                                   134 
135 A read of this file will get atomic_t values,     135 A read of this file will get atomic_t values, and a write of this file
136 will set atomic_t values.                         136 will set atomic_t values.
137                                                   137 
138 Another option is exporting a block of arbitra    138 Another option is exporting a block of arbitrary binary data, with
139 this structure and function::                     139 this structure and function::
140                                                   140 
141     struct debugfs_blob_wrapper {                 141     struct debugfs_blob_wrapper {
142         void *data;                               142         void *data;
143         unsigned long size;                       143         unsigned long size;
144     };                                            144     };
145                                                   145 
146     struct dentry *debugfs_create_blob(const c    146     struct dentry *debugfs_create_blob(const char *name, umode_t mode,
147                                        struct     147                                        struct dentry *parent,
148                                        struct     148                                        struct debugfs_blob_wrapper *blob);
149                                                   149 
150 A read of this file will return the data point    150 A read of this file will return the data pointed to by the
151 debugfs_blob_wrapper structure.  Some drivers     151 debugfs_blob_wrapper structure.  Some drivers use "blobs" as a simple way
152 to return several lines of (static) formatted     152 to return several lines of (static) formatted text output.  This function
153 can be used to export binary information, but     153 can be used to export binary information, but there does not appear to be
154 any code which does so in the mainline.  Note     154 any code which does so in the mainline.  Note that all files created with
155 debugfs_create_blob() are read-only.              155 debugfs_create_blob() are read-only.
156                                                   156 
157 If you want to dump a block of registers (some    157 If you want to dump a block of registers (something that happens quite
158 often during development, even if little such  !! 158 often during development, even if little such code reaches mainline.
159 debugfs offers two functions: one to make a re !! 159 Debugfs offers two functions: one to make a registers-only file, and
160 another to insert a register block in the midd    160 another to insert a register block in the middle of another sequential
161 file::                                            161 file::
162                                                   162 
163     struct debugfs_reg32 {                        163     struct debugfs_reg32 {
164         char *name;                               164         char *name;
165         unsigned long offset;                     165         unsigned long offset;
166     };                                            166     };
167                                                   167 
168     struct debugfs_regset32 {                     168     struct debugfs_regset32 {
169         const struct debugfs_reg32 *regs;         169         const struct debugfs_reg32 *regs;
170         int nregs;                                170         int nregs;
171         void __iomem *base;                       171         void __iomem *base;
172         struct device *dev;     /* Optional de    172         struct device *dev;     /* Optional device for Runtime PM */
173     };                                            173     };
174                                                   174 
175     debugfs_create_regset32(const char *name,     175     debugfs_create_regset32(const char *name, umode_t mode,
176                             struct dentry *par    176                             struct dentry *parent,
177                             struct debugfs_reg    177                             struct debugfs_regset32 *regset);
178                                                   178 
179     void debugfs_print_regs32(struct seq_file     179     void debugfs_print_regs32(struct seq_file *s, const struct debugfs_reg32 *regs,
180                          int nregs, void __iom    180                          int nregs, void __iomem *base, char *prefix);
181                                                   181 
182 The "base" argument may be 0, but you may want    182 The "base" argument may be 0, but you may want to build the reg32 array
183 using __stringify, and a number of register na    183 using __stringify, and a number of register names (macros) are actually
184 byte offsets over a base for the register bloc    184 byte offsets over a base for the register block.
185                                                   185 
186 If you want to dump a u32 array in debugfs, yo !! 186 If you want to dump an u32 array in debugfs, you can create file with::
187                                                   187 
188     struct debugfs_u32_array {                    188     struct debugfs_u32_array {
189         u32 *array;                               189         u32 *array;
190         u32 n_elements;                           190         u32 n_elements;
191     };                                            191     };
192                                                   192 
193     void debugfs_create_u32_array(const char *    193     void debugfs_create_u32_array(const char *name, umode_t mode,
194                         struct dentry *parent,    194                         struct dentry *parent,
195                         struct debugfs_u32_arr    195                         struct debugfs_u32_array *array);
196                                                   196 
197 The "array" argument wraps a pointer to the ar    197 The "array" argument wraps a pointer to the array's data and the number
198 of its elements. Note: Once array is created i    198 of its elements. Note: Once array is created its size can not be changed.
199                                                   199 
200 There is a helper function to create a device- !! 200 There is a helper function to create device related seq_file::
201                                                   201 
202    void debugfs_create_devm_seqfile(struct dev    202    void debugfs_create_devm_seqfile(struct device *dev,
203                                 const char *na    203                                 const char *name,
204                                 struct dentry     204                                 struct dentry *parent,
205                                 int (*read_fn)    205                                 int (*read_fn)(struct seq_file *s,
206                                         void *    206                                         void *data));
207                                                   207 
208 The "dev" argument is the device related to th    208 The "dev" argument is the device related to this debugfs file, and
209 the "read_fn" is a function pointer which to b    209 the "read_fn" is a function pointer which to be called to print the
210 seq_file content.                                 210 seq_file content.
211                                                   211 
212 There are a couple of other directory-oriented    212 There are a couple of other directory-oriented helper functions::
213                                                   213 
214     struct dentry *debugfs_rename(struct dentr    214     struct dentry *debugfs_rename(struct dentry *old_dir,
215                                   struct dentr    215                                   struct dentry *old_dentry,
216                                   struct dentr    216                                   struct dentry *new_dir,
217                                   const char *    217                                   const char *new_name);
218                                                   218 
219     struct dentry *debugfs_create_symlink(cons    219     struct dentry *debugfs_create_symlink(const char *name,
220                                           stru    220                                           struct dentry *parent,
221                                           cons    221                                           const char *target);
222                                                   222 
223 A call to debugfs_rename() will give a new nam    223 A call to debugfs_rename() will give a new name to an existing debugfs
224 file, possibly in a different directory.  The     224 file, possibly in a different directory.  The new_name must not exist prior
225 to the call; the return value is old_dentry wi    225 to the call; the return value is old_dentry with updated information.
226 Symbolic links can be created with debugfs_cre    226 Symbolic links can be created with debugfs_create_symlink().
227                                                   227 
228 There is one important thing that all debugfs     228 There is one important thing that all debugfs users must take into account:
229 there is no automatic cleanup of any directori    229 there is no automatic cleanup of any directories created in debugfs.  If a
230 module is unloaded without explicitly removing    230 module is unloaded without explicitly removing debugfs entries, the result
231 will be a lot of stale pointers and no end of     231 will be a lot of stale pointers and no end of highly antisocial behavior.
232 So all debugfs users - at least those which ca    232 So all debugfs users - at least those which can be built as modules - must
233 be prepared to remove all files and directorie    233 be prepared to remove all files and directories they create there.  A file
234 can be removed with::                             234 can be removed with::
235                                                   235 
236     void debugfs_remove(struct dentry *dentry)    236     void debugfs_remove(struct dentry *dentry);
237                                                   237 
238 The dentry value can be NULL or an error value    238 The dentry value can be NULL or an error value, in which case nothing will
239 be removed.                                       239 be removed.
240                                                   240 
241 Once upon a time, debugfs users were required     241 Once upon a time, debugfs users were required to remember the dentry
242 pointer for every debugfs file they created so    242 pointer for every debugfs file they created so that all files could be
243 cleaned up.  We live in more civilized times n    243 cleaned up.  We live in more civilized times now, though, and debugfs users
244 can call::                                        244 can call::
245                                                   245 
246     void debugfs_remove_recursive(struct dentr    246     void debugfs_remove_recursive(struct dentry *dentry);
247                                                   247 
248 If this function is passed a pointer for the d    248 If this function is passed a pointer for the dentry corresponding to the
249 top-level directory, the entire hierarchy belo    249 top-level directory, the entire hierarchy below that directory will be
250 removed.                                          250 removed.
251                                                   251 
252 .. [1] http://lwn.net/Articles/309298/            252 .. [1] http://lwn.net/Articles/309298/
                                                      

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